Zero Gravity Fluids

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Im trying to create some microgravity fluid sims within Houdini and have been running into a few issues. The shot is taking an astronauts bag/cup of Tang with the straw attached and squirting into another receptacle. The fluid is to collide with the walls of the container and slowly float up out of the container and out of frame. While this is happening both the original bag and the container are in the hands of two astronauts, so they are both moving constantly. The container the fluid is being shot into is a bowl with thick walls that kind of looks like a mortar with slightly deformed surface on the inner walls.

First issue was choosing which of Houdinis fluid solvers to choose. Im pretty sure I want to go with a particle fluid as I am very comfortable in POPs, and I know how to control the movement as such. I also do not want to introduce the "evaporating" sections that I have seen in volume fluid boxes when the fluid extends itself.

My initial idea was to use Flip fluids, both because I haven't used them all that much (so I could learn from the project) and because of the speed increase during sim time. I have been having issues trying to control the surface tension upon collisions and as I increase my sub-steps, obviously, the sim time increases dramatically.

Next I tried using SPH, but the surface tension acted the in a manner opposite of my expectations. Upon collision with the container (which I have simming as a static object) I believe, and I could be 100% incorrect, the surface tension tolerance is being met, so the surface tends to explode outward, even at 100+ sub-steps.

Im really here asking for any advice. I searched through the forum and could not find a post dealing with microgravity water, so if I missed a previous post, my apologies.

What solver would you use? Has this been done already and there is a write up that I am missing? Is surface tension the right way to go about this? Would you simply use pop forces to animate where you wish the particle to go and finally skin it with the fluid sim?

Any and all replies will be very very greatly appreciated!! Thanks in advance guys!!

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SPH fluids tend to explode a lot, that is as far as I know one of the main reasons for the development of flip fluids.

If I understand your problem correctly your are trying to get the fluids to look like they are sticking more together (not breaking off in hundreds of little drops) and you want to go for something like this:

If that is what you want then I would suggest to use viscosity. not a lot but just enough to make the particles "want" to stay together (values ranging form 200-500 might work nicely)

Then to control the direction of the particles and the exact path of them you can use a curve and push the particles a long of it using sop/pop solvers.

As for the collision with the container, since it is as you said animated I would highly recommend to use a point RBD object rather then a static object with deforming geometry turned on.

It is much faster and the collision geometry is more accurate since it does not recalculate it on every frame.

that should get you pretty far to start with and if the viscosity method is getting you the "surface tension look" that you want but it is getting rid of to many of the smaller motions of the water deforming, you can always add a vop force and make the flip fluids move through a curl noise and add that to the velocity.

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I was trying viscosity. Like you said it was removing a lot of the surface movement, and was forcing the sim to appear as though it was clay/dough.

I will try using the Vop force to stimulate the surface movement. Please pardon my ignornance on the subject, but how do I add the vop force to Flip? Do I add it to a pop for the creation of the Flip? Or do I add it as another force int he simulation?

I will also try using a point object for my collision geo. To be honest, didnt even think of doing that.

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It is very basic and I am not using curves or any of that to control the motion of the particles, but it shows the principals and idea behind it.

There are two easy ways of adding some curl noise or as a matter of fact what ever you want to a flip sim, using a Vop Force or a sop solver.

The Vop force, at least in my experience is deadly slow... especially when you start looking up values again each other and build in a few loops.

So I normally use a sop solver with a vop in side.

For this example I am simply adding some curl noise to the existing vel and feed that back out... the only thing extra that I am doing is to look up the current vel, and fit it into a rang form >0 to <=1 and multiply the curl noise by that.

That means that you will have more surface movement where the fluid moves faster and less where its slow, with out ever losing it completely.

Just set the values for the curl that you want and a max input for the fit rang and you should be good to go.